Metals | Issue Cover Articles Published in 2023

1. “Applications of Nanoporous Gold in Therapy, Drug Delivery, and Diagnostics”
by Palak Sondhi, Dhanbir Lingden, Jay K. Bhattarai, Alexei V. Demchenko and Keith J. Stine
Metals 2023, 13(1), 78; https://doi.org/10.3390/met13010078
Available online: https://www.mdpi.com/2075-4701/13/1/78

Cover Story: Nanoporous gold (np-Au) is emerging as an important nanomaterial in drug delivery, diagnostics and therapeutics. Np-Au offers a tunable pore size, biocompatibility and ease of fabrication into thin films, particles or shapes such as cylinders. The material can serve as a host for drug molecules with adjustable release for use in implanted devices. The plasmonic properties of np-Au enable applications in the photothermal therapy of cancer and also in diagnostic sensor development. Np-Au is an excellent electrode material for use in sensitive electrochemical detection schemes for biomarkers.

2. “Effects of B and Ce Grain Boundary Segregation on Precipitates in Super Austenitic Stainless Steel”
by Song Yang, Jinyao Ma, Chao Chen, Caili Zhang, Junyu Ren, Zhouhua Jiang, Guangwei Fan and Peide Han
Metals 2023, 13(2), 326; https://doi.org/10.3390/met13020326
Available online: https://www.mdpi.com/2075-4701/13/2/326

Cover Story: The precipitation of a Cr- and Mo-rich σ phase in super austenitic stainless steel (SASS) seriously affects the steel’s thermoplasticity and corrosion resistance. A new air-cooling and low-temperature diffusion treatment method was adopted, its purpose being to inhibit these precipitates. Ce inhibited the nucleation of the precipitates, while B induced the formation of Mo-rich precipitates and precipitates containing B and Mo, with the B-containing precipitates being serrated. The formation of B-containing serrated precipitates diminished the Cr-depleted zone near the grain boundary. The addition of B and Ce could lead to a significant improvement in the corrosion resistance of S31254 SASS.

3. “Effect of Laser Shock Peening on the Stress Corrosion Cracking of 304L Stainless Steel”
by Young-Ran Yoo, Seung-Heon Choi and Young-Sik Kim
Metals 2023, 13(3), 516; https://doi.org/10.3390/met13030516
Available online: https://www.mdpi.com/2075-4701/13/3/516

Cover Story: Laser shock peening (LSP) was performed on 304L stainless steel, and the SCC properties were evaluated via the U-bend test in 42% MgCl2. LSP induced a residual compressive stress in the depth direction and a grain refinement and an increased pitting potential of the cross section, and thus, the total and net crack propagation rates were reduced. Based on multiple regression analysis, the relationship between residual stress and crack propagation rates was strong, but the relationships between the average grain size and pitting potential of the cross section and crack propagation rates were moderate and weak, respectively.

4. “Study on the Bath Smelting Reduction Reaction and Mechanism of Iron Ore: A Review”
by Guilin Wang, Jianliang Zhang, Yaozu Wang, Yubo Tan, Zhen Li, Bo Zhang and Zhengjian Liu
Metals 2023, 13(4), 672; https://doi.org/10.3390/met13040672
Available online: https://www.mdpi.com/2075-4701/13/4/672

Cover Story: Bath smelting reduction technology is globally favored as a non-blast-furnace ironmaking technology that directly reduces iron ore into liquid metal without using coke as the raw material. This work focuses on three key aspects of smelting reduction, namely the thermal decomposition characteristics of iron ore during smelting reduction, the smelting reduction mechanism of iron-ore particles, and the smelting reduction mechanism of FeO-bearing slag. The experimental study methods, reaction mechanisms, influencing factors, and kinetic behavior of the three are highlighted to help foster a better understanding of the nature of the bath smelting reduction reaction.

5. “Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion”
by Federico Larini, Riccardo Casati, Silvia Marola and Maurizio Vedani
Metals 2023, 13(5), 924; https://doi.org/10.3390/met13050924
Available online: https://www.mdpi.com/2075-4701/13/5/924

Cover Story: A precipitation-hardening Al2139 alloy doped with additions of Zr and Ti was printed via laser powder bed fusion (L-PBF) and subsequently exposed to temperatures up to 300 °C. The relationship between performance degradation and changes in material structure was investigated through microhardness monitoring and microstructure analysis. The fully equiaxed fine-grained as-built state exhibits micro-segregations of Cu and Mg at the grain boundaries. These disappeared after the optimized T4 heat treatment, which formed rod-shaped Zr- and Ti-based second phases and quasi-spherical Cu-, Mn-, and Fe-rich particles. High-temperature holding leads to the rearrangement and subsequent spheroidization of Cu- and Mg-rich particles at the grain boundaries.

6. “Ranking of Injection Biochar for Slag Foaming Applications in Steelmaking”
by Christopher DiGiovanni, Delin Li, Ka Wing Ng and Xianai Huang
Metals 2023, 13(6), 1003; https://doi.org/10.3390/met13061003
Available online: https://www.mdpi.com/2075-4701/13/6/1003

Cover Story: EAF has the potential to decarbonize the steel industry. Carbon needs to be injected into molten slag for slag foaming in EAF steelmaking. Biocarbon is gaining attention as an alternative to fossil carbon. Two biochar candidates were tested in an induction furnace to simulate EAF conditions. The slag foaming height was measured and compared to fossil carbon candidates. The results show that biochar has considerable slag foaming capacity, specifically in bio-briquette forms. Ongoing research aims to develop a standardized methodology for ranking and qualifying different biochar candidates for larger-scale use.

7. “Study on the Wear Resistance of Ni-Co-ZrO₂ Composite Coatings with Different ZrO₂ Nanoparticle Concentrations Prepared Using Electrodeposition on the Micro-Surface of Spindle Hook Teeth”
by Tianxin Dong, Xingyu Wang, Fei Li, Yifan Zhu and Xiuqing Fu
Metals 2023, 13(7), 1251; https://doi.org/10.3390/met13071251
Available online: https://www.mdpi.com/2075-4701/13/7/1251

Cover Story: To improve the wear resistance of the surface of the cotton picker spindle, a Ni-Co-ZrO₂ composite coating doped with ZrO₂ nanoparticles was prepared via electrochemistry on the microsurfaces of spindle hook teeth. The effects of different ZrO₂ nanoparticle concentrations on various parameters of the coating were discussed. The test results indicated that Ni-Co-ZrO₂ composite coatings featured a more compact coating structure, a greater coating thickness, and a smaller grain size. The presence of ZrO₂ nanoparticles led to further improvements in the coating’s wear resistance. When the mass concentration of ZrO₂ nanoparticles reached 4 g/L, the wear resistance reached its highest level.

8. “Microstructural Characterization of Reactor Pressure Vessel Steels”
by Libang Lai, Paul Chekhonin, Shavkat Akhmadaliev, Jann-Erik Brandenburg and Frank Bergner
Metals 2023, 13(8), 1339; https://doi.org/10.3390/met13081339
Available online: https://www.mdpi.com/2075-4701/13/8/1339

Cover Story: Ion irradiation is a promising tool for emulating neutron-irradiation effects on reactor pressure vessel (RPV) steels. Comprehensive microstructure characterization was performed by means of different electron microscopies. The main work was based on scanning transmission electron microscopy applied to cross-sectional samples prepared using the focused ion beam technique to investigate ion irradiation-induced loops. Two different types of RPV steels with two levels of ion displacement damage were compared. Furthermore, a comparison was performed with the same samples previously irradiated with neutrons, and the results were discussed.

9. “Aging Behaviour of a 12.2Cr-10Ni-1Mo-1Ti-0.6Al Precipitation-Hardening Stainless Steel Manufactured via Laser Powder Bed Fusion”
by Alessandro Morri, Mattia Zanni, Lorella Ceschini, Alessandro Fortunato and Massimo Pellizzari
Metals 2023, 13(9), 1552; https://doi.org/10.3390/met13091552
Available online: https://www.mdpi.com/2075-4701/13/9/1552

Cover Story: The effect of solution annealing on the microstructure, aging, and over-aging behaviour of a precipitation-hardening stainless steel (PHSS) manufactured via laser powder bed fusion (LPBF) was investigated. Despite the different microstructures in the as-built and solution annealed conditions, the steel exhibited the same peak hardness, aging, and over-aging behaviour, which was related to nanometric precipitates and martensite-to-austenite reversion. Therefore, solution annealing on LPBF-manufactured PHSS is not mandatory in order to achieve high hardness and thermal stability, which are required for plastic injection moulding applications, thus enabling the production of complex tools with conformal cooling channels, hardened through simple direct aging treatment.

10. “Effect of Magnet Alternate Stirring on the Internal Quality of Sn-Pb Alloy”
by Mengyun Zhang, Yanping Bao and Haibo Zhang
Metals 2023, 13(10), 1732; https://doi.org/10.3390/met13101732
Available online: https://www.mdpi.com/2075-4701/13/10/1732

Cover Story: A permanent magnet stirrer was built to study the effect of different magnetic field stirring modes on the solidification quality of Sn-20 wt-% Pb alloy ingots. The internal quality of the ingot can be improved by adjusting both the stirring speed and the modes. In this paper, a permanent magnet stirrer was developed instead of an electromagnetic stirrer, and a Sn-20%Pb alloy was used instead of steel. Furthermore, the effects of different magnetic field stirring modes on the solidification structure and element uniformity of the alloy ingots were studied.

11. “Fast Gold Recovery from Aqueous Solutions and Assessment of Antimicrobial Activities of Novel Gold Composite”
by Tamara Tadić, Bojana Marković, Zorica Vuković, Plamen Stefanov, Danijela Maksin, Aleksandra Nastasović and Antonije Onjia
Metals 2023, 13(11), 1864; https://doi.org/10.3390/met13111864
Available online: https://www.mdpi.com/2075-4701/13/11/1864

Cover Story: The design and development of antibacterial polymer-supported materials, which can be easily functionalized, could improve their antimicrobial properties due to the synergistic effect between nanoparticles and the polymer matrix and solve the nanoparticle aggregation problem. In this study, a novel porous gold polymer composite was prepared via the functionalization of glycidyl methacrylate-based copolymers with ethylene diamine (pGME-en) and was demonstrated to be a rapid adsorbent for gold from aqueous solutions. Furthermore, the subsequent use of this versatile sorbent as an antimicrobial agent was investigated.

12. “Evaluation of the Applicability of Voltammetric Modes in Scanning Electrochemical Microscopy for In Situ Corrosion Characterisation of Copper-Based Materials”
by Brenda Hernández-Concepción, Adrián Méndez-Guerra, Ricardo M. Souto and Javier Izquierdo
Metals 2023, 13(12), 1965; https://doi.org/10.3390/met13121965
Available online: https://www.mdpi.com/2075-4701/13/12/1965

Cover Story: The introduction of scanning electrochemical microscopy (SECM) into the study of corrosion reactions and mechanisms provides interesting insights on initiation events and their dissemination with chemical and spatial resolution. More recently, its applicability has expanded thanks to methodological and instrumental developments that allow the combination of SECM with other spectroscopic, scanning probe microscopy, or electrochemical techniques to collect information on even earlier time scales and from systems with smaller differences in local chemical reactivity. This work shows the effectiveness of combining SECM with electrochemical voltammetry techniques to monitor in situ metal dissolution processes associated with the acidic corrosion of copper-based materials.